This invention relates to making the head space of aircraft fuel tanks inert to combustion and more specifically to a system and a process utilizing inert gas to reduce the oxygen content of the head space and the aircraft fuel in the tanks.
Recently a number of aircraft explosions have occurred which resulted in loss of life, injury to persons and extensive destruction of property. Cause for many of these explosions is attributed to the detonation and catastrophic combustion of fuel in the fuel tanks of the aircraft. The commercial aircraft industry, authorities that regulate the industry and users of commercial aircraft are concerned about safety from fuel tank explosions. There is a great need for technological advances to reducing the risk of this hazard.
In operation, aircraft fuel tanks contain a liquid inventory of aircraft fuel and a vapor composition in the space within the tank not occupied by the liquid fuel. This space is often referred to as the xe2x80x9cullagexe2x80x9d or xe2x80x9cthe head spacexe2x80x9d of the tank. Oxygen mixed with fuel vapor is usually present in the head space. One likely cause of fuel tank explosions is the simultaneous combination of an explosively combustible mixture of oxygen and fuel vapor in the head space and a source of ignition. Such ignition sources are due to accidental fire, sparking due to faulty or degraded electrical system components, static electricity discharge, or energy suddenly released on impact by collision with an object, for example. If the oxygen-fuel vapor explodes, it is likely to destroy the integrity of the tank and thereby release more fuel to exacerbate the disaster.
One way to negate the possibility of an explosively combustible oxygen-fuel vapor mixture forming in an aircraft fuel tank is to prevent the concentration of oxygen in the head space from exceeding the minimum limits for flammability. Oxygen can enter the head space in gaseous form when fuel is consumed by the aircraft engines. That is, as the fuel is consumed, the level of liquid fuel in the tank is lowered, which draws in ambient air containing about 21 vol. % oxygen from outside the tank to fill the void created by the vacated fuel.
Oxygen can also enter the tank with the fuel. For example, oxygen dissolves in the fuel when the fuel is stored in vented storage tanks at an airport prior to filling the aircraft fuel tank. That is, due to vapor-liquid equilibrium, some of the oxygen present in the atmosphere above the fuel diffuses and dissolves into the liquid phase. In flight, the local ambient pressure drops due to the change of aircraft altitude. The vapor liquid equilibrium shifts to favor liberation of substantial amounts of the dissolved oxygen into the head space vapor as the pressure goes down.
Oxygen concentration of the fuel tank head space can be reduced by initially purging it from the tank and replenishing the volume voided during fuel consumption with an inert gas, e.g., nitrogen, carbon dioxide, argon, and others. The procedure of displacing the vapor in the head space with one of another concentration or composition is sometimes referred to herein as xe2x80x9cullage washingxe2x80x9d or xe2x80x9chead space inertingxe2x80x9d. Also, the liquid fuel can be purged to very low concentrations of oxygen prior to filling the tank. The latter process typically involves contacting the liquid fuel with large quantities of inert gas. The dissolved oxygen redistributes between the liquid and the low oxygen content scrubbing gas which is swept away leaving less oxygen in the fuel. The procedure of removing dissolved oxygen from liquid fuel is sometimes referred to herein as xe2x80x9cfuel scrubbingxe2x80x9d or xe2x80x9cfuel deoxygenationxe2x80x9d.
Conventional cryogenic methods of producing sufficient quantities of oxygen-free or nearly free inert gas for fuel deoxygenation and/or head space washing operate at extreme temperatures and pressures. They usually utilize large, heavy, complex and often noisy machinery that requires high power to operate. Cryogenic inert gas production facilities are also usually expensive. These factors normally promote location of such facilities remotely from commercial aircraft fuel tank dispensing sites and airport passenger terminal buildings. Cryogenic inert gas production typically results in a highly pressurized inert material in liquid form, for example, liquid nitrogen, which is stored at well below ambient temperature. Body contact with liquid nitrogen can cause serious personal injury.
It would therefore be desirable to provide a method by which inert gas, preferably nitrogen, can be conveniently and efficiently supplied to aircraft fuel tank dispensing sites for head space washing and/or fuel deoxygenation operations in a manner that eliminates problems associated with cryogenic inert gas, especially cryogenic nitrogen.
Head space inerting can involve exhausting the gas in equilibrium with the liquid fuel inventory from aircraft fuel tank. Such gas includes volatile organic components of fuel vapor which pollutes the ambient atmosphere and thereby creates risk of damage to health of personnel in the area and to the environment. Hence, it is desirable to provide an aircraft fuel tank inerting system that produces significantly less air pollution than conventional practices.
This invention provides a system comprising a variety of elements which as a unit provide the ability to produce and maintain low concentrations of oxygen in aircraft fuel tank head space and liquid fuel conveniently, environmentally safely and relatively economically. The reduced oxygen concentrations are effective to significantly reduce the explosion potential of the head space during aircraft operation. The system is based on the ground and features an inert gas distribution subsystem adapted to dispense head space inerting gas to each aircraft boarding/loading location at an airport.